Update README.rst
Update README.rst

Link to https://certbot.eff.org/ was broken
jsha/letsencrypt,stweil/letsencrypt,jsha/letsencrypt,bsmr-misc-forks/letsencrypt,jtl999/certbot,lmcro/letsencrypt,stweil/letsencrypt,bsmr-misc-forks/letsencrypt,jtl999/certbot,letsencrypt/letsencrypt,letsencrypt/letsencrypt,lmcro/letsencrypt